Lactose, milk sugar, is digested by intestinal lactase. Persons with low levels of lactase activity cannot adequately digest a large lactose load, equivalent to 3 or 4 glasses of milk. Blood sugar rises are low and they note abdominal symptoms with the test ("lactose intolerant"). Our pilot studies had shown racial and ethnic differences in the prevalence of lactose intolerance. It is now known that lactose intolerance is probably present in the majority of the world's teenage and adult population. The age of onset in children varies in different ethnic groups. Our preliminary studies of 40 elementary school age Negro children suggest that at least 40% are intolerant of a large lactose load equivalent to 3 or 4 glasses of milk. Ten to 15% of U.S. white school children and adults are intolerant. Although it is becoming clear that lactose intolerance is common in many healthy children, further information on the clinical and nutritional significance of a person being intolerant of a large tolerance test dose of lactose is needed for proper medical and nutritional planning. The tolerance of commonly consumed amounts of milk and of lactose in children intolerant of large loads will be evaluated. The ability of questioning of awareness of milk intolerance to predict intolerance of small amounts of milk will be assessed. The absorption of milk components and of other dietary materials ingested with milk will be tested. Interrelationships of galactose metabolizing systems with lactose digestion will be examined. Methods of increasing milk and lactose tolerance will be evaluated because of the possible nutritional implications of milk intolerance. The pattern of postnatal development of intestinal lactase activity in surgical and autopsy specimens of mucosa will be determined in a population with a high prevalence of lactose intolerance in adulthood (Negroes). A relationship between postnatal patterns and milk ingestion will be sought. Association of genetic markers with lactose intolerance will be examined. Pre-hydrolyzed milk will be evaluated.